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Chapter 36. Preterm Birth

Low birthweight defines neonates who are born too small. Preterm or premature births are terms used to define neonates who are born too early. With respect to gestational age, a newborn may be preterm, term, or postterm. With respect to size, a newborn may be normally grown and appropriate for gestational age; small in size, thus, small for gestational age; or overgrown and consequently, large for gestational age. In recent years, the term small for gestational age has been widely used to categorize newborns whose birthweight is usually below the 10th percentile for gestational age. Other frequently used terms have included fetal-growth restriction or intrauterine growth restriction. The term large for gestational age has been widely used to categorize newborns whose birthweight is above the 90th percentile for gestational age. The term appropriate for gestational age designates newborns whose weight is between the 10th and 90th percentiles. Thus, infants born before term can be small or large for gestational age but still fit the definition of preterm. Low birthweight refers to births 500 to 2500 g; very low birthweight refers to births 500 to 1500 g; and extremely low birthweight refers to births 500 to 1000 g. In 1960, a neonate weighing 1000 g had a 95-percent risk of death. Today, a neonate with the same birthweight has a 95-percent chance of surviving (Ingelfinger, 2007). This remarkable improvement in survival is due to the widespread application of neonatal intensive care in the early 1970s.

In the United States in 2005, 28,384 infants died in their first year of life (Table 36-1). Preterm birth, which is defined as delivery before 37 completed weeks, was implicated in approximately two thirds of these deaths. As shown in Table 36-1, late preterm births, defined as those 34 to 36 weeks' gestation, composed approximately 70 percent of all preterm births. As discussed in Chapter 1, Infant Deaths, although the infant mortality rate for the United States has declined substantively over the past century, it has remained static from 2000 to 2005 (MacDorman and Matthews, 2008). Thus, the issue of preterm birth remains a major health problem.

The rates of preterm birth—the largest contributor to infant mortality—began to increase in the United States in 1996. As shown in Figure 36-1, medically indicated preterm births are largely responsible for this ...